The interest in structures having dimensions sufficiently small to lead to quantum confinement effects has continued unabated since the initial demonstration of one-dimensional quantum wells in GaAs. The ability to extend the confinement to three dimensions in the form of quantum dots of CdS, CdSe, and CdTe in glasses has sparked further research activity. Among the methods for the preparation of quantum dots reported in the literature have been the formation and deposition in porous glasses, the chemical preparation and subsequent suspensions in an organic or polymer matrix, and sol-gel based methods. The advantage one method may have over another is in its ability to produce a narrower particle size distribution and/or a more defect-free crystallite phase. The latter condition is difficult to produce because, for the above-mentioned materials at the size required for confinement (as explained hereinafter), the surface contribution of the crystallite, whether it be density of states or defects, compares favorably to the volume contribution. The idealized theoretical treatments cannot take into account the surface effects. That circumstance suggests the extension of the quantum dot development in glass to materials which have significantly smaller effective masses so that quantum confinement would be achieved at larger crystallite sizes and, hence, minimize surface contributions.
The crystal compositions that have been prepared via one or more of the above-mentioned methods have involved the following crystal systems: (1) materials from the I and VII group of the Periodic Table such as CuCl and CuBr; (2) materials from the II and VI groups of the Periodic Table such as CdS, ZnS, and CdSe; and (3) materials from the IV and VI groups of the Periodic Table such as PbS and PbSe.
A. L. Efros et al., Sov. Phys.--Semicond. 16, 772 (1992), appear to be the first to classify the confinement with respect to the electron and hole Bohr radii, a.sub.e = .sup.2 /m.sub.2 2.sup.2 and a.sub.h = .sup.2 /m.sub.h 2.sup.2, respectively, as compared to the crystallite radius a. In those two expressions is the optical dielectric constant, e is the electronic charge, is Planck's constant, and m is the respective effective mass of the electron and the hole. The classification is illustrated in Table I below, along with the theoretical energy shift and the most likely materials that satisfy the confinement criteria. In the strong and intermediate confinement cases, the experimental effect observed is a blue shift of the fundamental absorption edge accompanied by the onset of absorption bands corresponding to the discrete energy states produced by the confinement. The resolution or sharpness of these bands depends upon the narrowness of the particle size distribution and the inherent homogeneous line width. For the weak confinement case the experimental result is a shift in the energy of the exciton. This action can be viewed as the effect of the confinement of the exciton itself. In the strong and intermediate confinement cases the quasi-particle is the electron and the hole, whereas in the weak confinement case it is the exciton.
TABLE I ______________________________________ Crystallite Energy* Confinement Size Shift Possible Example ______________________________________ Strong a &lt; a.sub.e,a.sub.h .eta..sup.2 /2a.sup.2 .mu. PbS a.sub.2 = a.sub.h = 9 nm Intermediate a.sub.2 &lt; a &lt; a.sub.h .eta..sup.2 /2a.sup.2 m.sub.e CdSe a.sub.e = 3 nm a.sub.h = 0.5 nm Weak a &gt; a.sub.e,a.sub.h .eta..sup.2 /2a.sup.2 M CuCl a.sub.e,a.sub.h = 0.5 nm wherein 1/.mu. = 1/m.sub.e + 1/m.sub.h and M = m.sub.e + m.sub.h. ______________________________________ *Calculated as a particle in an infinite spherical well with the coulombi interaction ignored in accordance with M. Nogami et al., J. Am. Cer. Soc. 73, 2097 (1990).
Both the intermediate and weak cases have been observed in glass with the controlled crystallization of such microcrystalline phases as CuCl and CuBr in the former case [M. Nogami et al., J. Non-Cryst. Sol. 122, 101 (1990)], and CdS and CdSe in the latter [R. Klann et al., Appl. Phys. Lett., 59, (8), 885 (1991)].
To the knowledge of the present applicants there has been no report of the crystallization of PbS or PbSe directly from glass, although they have been prepared through chemical reactions in polymers, in porous glass, and in organic glasses. Each of those studies has demonstrated the occurrence of significant shifts in absorption edge, thereby indicating particle diameters on the order of 2-20 nm. The adsorption spectra, however, did not illustrate well-resolved, discrete adsorption peaks, as had been observed in the CdS and CdSe systems. That inspection of the adsorption spectra led to the conclusion that the particle size distribution was broad. We conjectured that, perhaps, by thermally developing in situ a PbS or PbSe crystal phase in a glass, a narrower particle size distribution could be obtained. In addition, there would be advantages in having the PbS or PbSe crystallites in an inorganic glass from the standpoint of having the crystalline phase in an inert, hermetic, and high temperature matrix.
Therefore, the principal objective of the present invention was to produce glass articles containing microcrystals of PbS and/or PbSe crystallized in situ therein.
A necessary companion objective was to devise a method for making such crystal-containing, glass articles.
A specific objective was to produce glass articles containing microcrystals of PbS and/or PbSe wherein the crystal dimensions are relatively uniform in size.